My recent experience cruising among the 176 islands of the Island Nation of Tonga has sparked an interest in better understanding portable navigation systems. But before getting into that, it helps to understand the Global Positioning System as it relates to your iOS device.

The iPhone and the 3G versions of the iPad have a specialized GPS chip that allows them to receive signals from NAVSTAR, the US global satellite navigation system. The iPhone and other iOS devices can also ascertain their position from nearby Wi-Fi and cellular telephone networks where available. The satellite-based system works well outdoors in just about any locale. Using Wi-Fi to fix your position is effective in areas where Wi-Fi hotspots are widely available, especially in densely populated urban and suburban locations. Cell tower triangulation works well in locations with significant cell tower penetration. Satellite-based GPS is by far the most accurate and most available of the three.

Navigating at sea

When sailing close to land, you may be able to triangulate on cell towers. But because cell towers and Wi-Fi hotspots are unavailable at sea, we rely on satellite-based GPS. The simplest way to do this with your iPhone is to use the iPhone's Compass app (or a similar third-party app) to get your latitude/longitude coordinates and then plot them on a chart. You can also hit the arrow icon in the lower left corner of the Maps app to display your current position on a map. However, the Maps app doesn't give you very much detail. You're better off with a more feature-rich program. Fortunately, a number of highly regarded apps are available for marine navigation.

Heading for the Island Nation of Tonga

I bought my iPhone four days before joining friends aboard their sailboat for two weeks of sailing in Tonga, a remotely located island nation in the South Pacific. Of Tonga's 176 islands, 36 are permanently inhabited and a number of the others have seasonally-used fishing camps.

Before leaving for Tonga, I installed the Navionics GPS program ($14.99, app2.me/3407) and charts for the South Pacific from the App Store. In addition, I installed GoSkyWatch ($5.99, app2.me/2985). The latter is a virtual astronomy app that orients itself to your coordinates and the direction you're pointing the iPhone, and displays the heavens accordingly.

I used the Navionics GPS program with South Pacific charts (left) and GoSkyWatch (right).

Experienced sailors will always have current, government-issued paper charts on board as a backup, but they increasingly rely on electronic navigational systems for day-to-day navigation. PC-based charting systems are used like a paper chart to understand local waters. They can also be integrated with a GPS receiver to display one's current position and track progress. These systems and the e-chart kits they use can be quite expensive. For example, e-chart kits can cost upwards of $350 for each region; depending on the vendor, there can be many regional e-charts.

Transforming the iPhone into a marine navigation system

Navigation systems for the iPhone, iPad, and other mobile devices are much more versatile and less expensive. Of course, the iPhone has a smaller screen than the iPad (or an external monitor). In some situations this is a hindrance, in others very helpful. As mentioned, the iPhone has basic GPS capability built into it. It is relatively inexpensive to convert it into a powerful navigation platform. The app does not have as many features as PC-based navigation applications, but it's catching up fast. Some of the PC-based features are missing, but some new and exciting features found in the app are not yet available in the PC products. It is truly an evolving product line.

Navionics made a major change to the platform in early December. In its first iteration, each version included the navigation app and full charts for a specific region of the ocean. (e.g., Europe, South America, etc.). The latest versions include the navigation application and chart outlines for the specific region without navigational or topographical data. Land is typically displayed as yellow with just a few cities as orientation landmarks; the ocean is grey (not blue) and it displays islands. Once installed, you tap on "Menu" in the lower right corner of the app to display a screen of tutorials and help items. Select the "Download Map" option on this screen to display the chart. Finally, you use the cropping tool to import detailed information about the area you've selected. There is an advantage to this approach: The app and charts don't occupy as much storage space on your iPhone. The disadvantage: You need a Web connection to download the detailed chart sections. Of course, a Web connection might not be available when you're at sea. To avoid problems, think ahead and download detailed info for all the areas you might possibly visit.

Most cruising boats already have a navigational computer onboard, but it's good to have a backup. While we sailed through the Ha'apai group of islands on our way to Tonga, our boat's primary navigational computer failed. My iPhone guided us the rest of the way and proved tremendously useful, especially as we entered the narrow channel leading to the shallow lagoon at Kelefesia. Of course, we had an experienced helmsman and capable lookouts on either side of the boat, but the iPhone made our passage much less threatening.

Our boat at home has two reliable GPS units that are 3-4 years old. However, neither is as easy to use or has as many capabilities as the Navionics/iPhone combo. I'm fully content to use my iPhone as my primary navigation system when sailing along the coast.

Rescue, ShipFinder, and other useful apps

GPS navigation is not the only solution you should look at. I found a number of interesting and useful apps that can come in handy on the high seas or closer to shore.

RESCUE ($2.99, app2.me/3408) transmits an emergency call via the cellular network and the lat/long coordinates via SMS using a 3G or EDGE network. Of course, you have to be close to port to use it.

RESCUE (left) and Ship Finder (right).

Ship Finder (free, app2.me/3276; other versions available) provides Automatic Identification System (AIS) information for ships within 40 miles of shore. AIS is a tremendously valuable source of location information for commercial and recreational ships and airplanes. Ships and planes use transceivers to send and receive signals. Non-commercial ships, smaller vessels, and private planes can use a relatively inexpensive receiver to monitor oncoming traffic routes to avoid collisions. Note that Military ships have AIS, but do not usually turn it on. Hence, they will not display on screen. One must keep a vigilant watch when in military zones, like Camp Pendleton, San Diego, etc. In addition, military ships may cancel out radar. So, if it is foggy and you are depending upon radar, be aware that military ships may be emitting radar-cancelling signals.

NOAA Buoy and Tide Data ($1.99, app2.me/3274) displays the location of government and research buoys. Clicking on any one will display specific information of value to navigators. Verona Solutions captures data regarding the ocean (wave height, period, weather, etc.) and makes it all available via this app. Of course, you can get this information on a Web-connected PC or via a telephone, but having the tables and charts to visualize this information via a handheld portable device is a tremendous aid to navigation.

Knot Guide ($1.99, app2.me/2326; free version, app2.me/3275) has a huge database of knots and their nautical and terrestrial uses as well as images showing you how to tie and use. (If it could only tie a bowline for me while I'm steering.)

NOAA Buoy and Tide Data (left), Knotguide (right)

RadarScope ($9.99, app2.me/2897) lets you view radar weather data from NEXRAD radar sites in the lower 48 US states and Puerto Rico. The app has an excellent support screen that will help you understand weather radar better.

Although I did not have time to review them, I recommend taking a look at the apps from Jeppeson and Navimatics. Jeppeson is famous for its aeronautical products and their C-Map charting system. The recently introduced CrewAlert ($19.99, app2.me/3409), an app that improves safety by monitoring crew alertness. Although it was developed to monitor airline crews, it's of use in just about any workplace environment. Navimatics offers a number of well-designed apps, including Charts & Tides ($24.99, app2.me/3414); Celestial, a digital sextant ($24.99, app2.me/3410); and Aero Charts (CONUS version: $24.99, app2.me/3411, Alaska version: $39.99, app2.me/3412).

Protect your investment

To protect my device, I use an anti-glare screen protector and the rubber ringed frame that Apple sent to iPhone 4 users for free. I drilled a 1/32" hole in the rubber frame 1/2' to the left of the lower left speaker opening, avoiding the seam in the frame. I then looped a camera wrist strap through the hole. Using a wrist strap is a great way to avoid dropping your iPhone.

I also bought a belt case to carry the iPhone when I'm dressed casually. Because I've had bad experiences with clip-on cases, I was careful to get one with a firmly sewn belt loop. I also use a felt-lined, slide-in eyeglass case to protect my iPhone when I wear a sport jacket. I didn't limit myself to a Web search or to cases that are made specifically for the iPhone. Instead, I visited specialty stores like Bass Outdoors and West Marine and looked through equine tack shops, hardware stores, and camera stores. This approach may take a little longer, but patience pays off.

Taking to the high seas has become more convenient with the iPhone. Now, you not only know where you are, you also have up-to-date information that can make your voyage safer.